Battery pack

ABSTRACT

An electrical combination including a driver drill capable of producing an average current draw of approximately 20-amps, a circular saw capable of producing an average current draw of approximately 20-amps, and a power tool battery pack operable to supply power to the driver drill and to the circular saw, the battery pack including a plurality of battery cells, the plurality of battery cells each having a lithium-based chemistry.

RELATED APPLICATIONS

The present patent application claims the benefits of prior filedco-pending U.S. provisional patent application Ser. No. 60/582,138,filed on Jun. 22, 2004; Ser. No. 60/582,728, filed on Jun. 24, 2004;Ser. No. 60/582,730, filed on Jun. 24, 2004; Ser. No. 60/612,352, filedon Sep. 22, 2004; Ser. No. 60/626,013, filed on Nov. 5, 2004; Ser. No.60/626,230, filed on Nov. 9, 2004 and Ser. No. 60/643,396, filed on Jan.12, 2005, the entire contents of all of which are hereby incorporated byreference.

The present patent application also claims the benefit of prior filedco-pending U.S. patent application Ser. No. 10/720,027, filed on Nov.20, 2003, which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/428,358, filed on Nov. 22,2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No. 60/428,452,filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan. 17, 2003;Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No. 60/523,716, filedon Nov. 19, 2003; and Ser. No. 60/523,712, filed on Nov. 19, 2003, theentire contents of all of which are hereby incorporated by reference.

The present patent application also claims the benefit of prior filedco-pending U.S. patent application Ser. No. 10/719,680, filed on Nov.20, 2003, which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/428,358, filed on Nov. 22,2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No. 60/428,452,filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan. 17, 2003;Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No. 60/523,716, filedon Nov. 19, 2003; and Ser. No. 60/523,712, filed on Nov. 19, 2003, theentire contents of all of which are hereby incorporated by reference.

The present patent application also claims the benefit of prior filedco-pending U.S. patent application Ser. No. 10/721,800, filed on Nov.24, 2003, which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/428,356, filed on Nov. 22,2002; Ser. No. 60/428,358, filed on Nov. 22, 2002; Ser. No. 60/428,450,filed on Nov. 22, 2002; Ser. No. 60/428,452, filed on Nov. 22, 2002;Ser. No. 60/440,692, filed on Jan. 17, 2003; Ser. No. 60/440,693, filedon Jan. 17, 2003; Ser. No. 60/523,712, filed on Nov. 19, 2003; and Ser.No. 60/523,716, filed on Nov. 19, 2003, the entire contents of all ofwhich are hereby incorporated by reference.

The present patent application also claims the benefit of prior filedco-pending U.S. patent application Ser. No. 11/138,070, filed on May 24,2005, which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/574,278, filed on May 24,2004; Ser. No. 60/574,616, filed on May 25, 2004; Ser. No. 60/582,138,filed on Jun. 22, 2004; Ser. No. 60/582,728, filed on Jun. 24, 2004;Ser. No. 60/582,730, filed on Jun. 24, 2004; Ser. No. 60/612,352, filedon Sep. 22, 2004; Ser. No. 60/626,013, filed on Nov. 5, 2004; Ser. No.60/626,230, filed on Nov. 9, 2004 and Ser. No. 60/643,396, filed on Jan.12, 2005, and prior filed co-pending U.S. patent application Ser. No.10/720,027, filed on Nov. 20, 2003 (which claims the benefits of priorfiled co-pending U.S. provisional patent application Ser. No.60/428,358, filed on Nov. 22, 2002; Ser. No. 60/428,450, filed on Nov.22, 2002; Ser. No. 60/428,452, filed on Nov. 22, 2002; Ser. No.60/440,692, filed on Jan. 17, 2003; Ser. No. 60/440,693, filed on Jan.17, 2003; Ser. No. 60/523,716, filed on Nov. 19, 2003; and Ser. No.60/523,712, filed on Nov. 19, 2003); Ser. No. 10/719,680, filed on Nov.20, 2003 (which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/428,358, filed on Nov. 22,2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No. 60/428,452,filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan. 17, 2003;Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No. 60/523,716, filedon Nov. 19, 2003; and Ser. No. 60/523,712, filed on Nov. 19, 2003); andSer. No. 10/721,800, filed on Nov. 24, 2003 (which claims the benefitsof prior filed co-pending U.S. provisional patent application Ser. No.60/428,356, filed on Nov. 22, 2002; Ser. No. 60/428,358, filed on Nov.22, 2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No.60/428,452, filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan.17, 2003; Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No.60/523,712, filed on Nov. 19, 2003; and Ser. No. 60/523,716, filed onNov. 19, 2003), the entire contents of all of which are herebyincorporated by reference.

The present patent application also claims the benefit of prior filedco-pending U.S. patent application Ser. No. 11/139,020, filed on May 24,2005, which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/574,278, filed on May 24,2004; Ser. No. 60/574,616, filed on May 25, 2004; Ser. No. 60/582,138,filed on Jun. 22, 2004; Ser. No. 60/582,728, filed on Jun. 24, 2004;Ser. No. 60/582,730, filed on Jun. 24, 2004; Ser. No. 60/612,352, filedon Sep. 22, 2004; Ser. No. 60/626,013, filed on Nov. 5, 2004; Ser. No.60/626,230, filed on Nov. 9, 2004 and Ser. No. 60/643,396, filed on Jan.12, 2005, and prior filed co-pending U.S. patent application Ser. No.10/720,027, filed on Nov. 20, 2003 (which claims the benefits of priorfiled co-pending U.S. provisional patent application Ser. No.60/428,358, filed on Nov. 22, 2002; Ser. No. 60/428,450, filed on Nov.22, 2002; Ser. No. 60/428,452, filed on Nov. 22, 2002; Ser. No.60/440,692, filed on Jan. 17, 2003; Ser. No. 60/440,693, filed on Jan.17, 2003; Ser. No. 60/523,716, filed on Nov. 19, 2003; and Ser. No.60/523,712, filed on Nov. 19, 2003); Ser. No. 10/719,680, filed on Nov.20, 2003 (which claims the benefits of prior filed co-pending U.S.provisional patent application Ser. No. 60/428,358, filed on Nov. 22,2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No. 60/428,452,filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan. 17, 2003;Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No. 60/523,716, filedon Nov. 19, 2003; and Ser. No. 60/523,712, filed on Nov. 19, 2003); andSer. No. 10/721,800, filed on Nov. 24, 2003 (which claims the benefitsof prior filed co-pending U.S. provisional patent application Ser. No.60/428,356, filed on Nov. 22, 2002; Ser. No. 60/428,358, filed on Nov.22, 2002; Ser. No. 60/428,450, filed on Nov. 22, 2002; Ser. No.60/428,452, filed on Nov. 22, 2002; Ser. No. 60/440,692, filed on Jan.17, 2003; Ser. No. 60/440,693, filed on Jan. 17, 2003; Ser. No.60/523,712, filed on Nov. 19, 2003; and Ser. No. 60/523,716, filed onNov. 19, 2003), the entire contents of all of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to battery packs and, moreparticularly, to power tool battery packs.

BACKGROUND OF THE INVENTION

Typically, electrical equipment, such as, for example, a cordless powertool, is powered by a rechargeable battery. The battery may beperiodically charged in a compatible battery charger.

SUMMARY OF THE INVENTION

FIGS. 36-38 illustrate an existing battery pack 230. The existingbattery pack 230 includes a housing 242 and at least one rechargeablebattery cell 246 (shown in FIGS. 39-40) supported by the housing 242. Inthe illustrated construction, the existing battery pack 230 is an 18Vbattery pack including (see FIGS. 39-40) fifteen approximately 1.2Vbattery cells 246 connected in series. The battery cells 246 are arechargeable battery cell chemistry type, such as, for example, NiCd orNiMH.

As shown in FIGS. 39-40, in the existing battery pack 230, each batterycell 246 is generally cylindrical and extends along a cell axis 250parallel to the cylindrical outer cell wall. In the existing batterypack 230, the cell axes 250 are parallel to one another. Also, in theexisting battery pack 230, each battery cell 246 has a cell length 252which is about two times the cell diameter 254. In the illustratedconstruction, each battery cell 246 has a length of about forty-sixmillimeters (46 mm) and a diameter of about twenty-three millimeters (23mm).

The existing battery pack 230 is connectable to (see FIG. 12) a batterycharger 38, and the battery charger 38 is operable to charge theexisting battery pack 230. The existing battery pack 230 is connectableto electrical equipment, such as, for example, a power tool 34 (shown inFIG. 11A), to power the power tool 34. As shown in FIGS. 36-38, thehousing 242 provides a support portion 250 for supporting the existingbattery pack 230 on an electrical device. In the illustratedconstruction, the support portion 250 provides (see FIG. 36) a C-shapedcross section which is connectable to a complementary T-shaped crosssection support portion on the electrical device (the support portion onthe power tool 34 (shown in FIG. 11B) and/or the battery support portionon the battery charger 38 (shown in FIG. 12).

The existing battery pack 230 includes (see FIGS. 36-37 and 39-40) aterminal assembly 286 operable to electrically connect battery cells 246to a circuit in the electrical device. The terminal assembly 286includes a positive battery terminal 298, a ground terminal 302, and asense terminal 306. As illustrated in FIGS. 39-40, the terminals 298 and302 are connected to the opposite ends of the cell or series of cells246. The sense terminal 306 is connected to (see FIG. 40) an electricalcomponent 314 which is connected in the circuit of the existing batterypack 230. In the illustrated construction, the electrical component 314is a temperature-sensing device or thermistor to communicate thetemperature of the existing battery pack 230 and/or of the battery cells246.

The present invention provides a battery pack which substantiallyalleviates one or more independent problems with the above-described andother existing battery packs. In some aspects and in some constructions,the present invention provides a battery pack including two cells whichare positioned in non-parallel relation to each other. In some aspects,the two cells are positioned in normal relation to each other.

More particularly, in some aspects and in some constructions, thepresent invention provides a battery pack including a housing, a firstcell extending along a first cell axis, and a second cell extendingalong a second cell axis, the first cell and the second cell beingsupported by the housing in an orientation in which the first cell axisis non-parallel to the second cell axis. In some aspects and in someconstructions, the first cell axis is normal to the second cell axis.

Also, in some aspects and in some constructions, the present inventionprovides a method of assembling a battery pack, the method including theacts of providing a battery pack housing, supporting a first cell withthe housing, and supporting a second cell with the housing innon-parallel relation to the first cell. In some aspects, the act ofsupporting the second cell includes supporting the second cell in normalrelation to the first cell.

In addition, in some aspects and in some constructions, the presentinvention provides a battery pack including a plurality of cells, asensor for sensing the voltage of a first group of the plurality ofcells, a sensor for sensing the voltage of a second group of theplurality of cells, and a controller for comparing the voltage of thefirst group to the voltage of the second group to determine if one ofthe plurality of cells is at or below a voltage.

Further, in some aspects and in some constructions, the presentinvention provides a method of determining a voltage of a cell of abattery pack, the battery pack including a plurality of cells, themethod including the acts of sensing the voltage of a first group of theplurality of cells, sensing the voltage of a second group of theplurality of cells, and comparing the voltage of the first group to thevoltage of the second group to determine if one of the plurality ofcells is at or below a voltage.

Also, in some aspects and in some constructions, the present inventionprovides a battery pack including a housing, a cell supported by thehousing, a FET connected to the cell, and a heat sink in heat-transferrelationship with the FET.

In addition, in some aspects and in some constructions, the presentinvention provides a method of assembling a battery pack, the methodincluding the acts of providing a housing, supporting a cell with thehousing, supporting a FET with the housing, connecting the FET to thecell, and supporting a heat sink in heat-transfer relationship with theFET.

Further, in some aspects and in some constructions, the presentinvention provides a battery including a housing supportable by anelectrical device, a cell supported by the housing and connectable tothe electrical device and a locking assembly for locking the battery tothe electrical device. The locking assembly includes a locking membersupported by the housing for movement between a locked position, inwhich the battery is locked to the electrical device, and an unlockedposition, an actuator supported by the housing and operable to move thelocking member between the locked position and the unlocked position anda biasing member operable to bias the locking member to the lockedposition, the biasing member being fixed between the actuator and thehousing and retaining the actuator in a position relative to thehousing.

Independent features and independent advantages of the invention willbecome apparent to those skilled in the art upon review of the detaileddescription and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front perspective view of a battery.

FIG. 2 is a top rear perspective view of the battery pack shown in FIG.1.

FIG. 3 is a bottom rear perspective view of the battery pack shown inFIG. 1.

FIG. 4 is a top view of the battery pack shown in FIG. 1.

FIG. 5 is a bottom view of the battery pack shown in FIG. 1.

FIG. 6 is a front view of the battery pack shown in FIG. 1.

FIG. 7 is a rear view of the battery pack shown in FIG. 1.

FIG. 8 is a right side view of the battery pack shown in FIG. 1.

FIG. 9 is a left side view of the battery pack shown in FIG. 1.

FIG. 10 is a bottom view of an alternate construction of a battery packembodying aspects of the present invention.

FIG. 11A is a perspective view of an electrical device, such as a powertool, for use with the battery pack shown in FIG. 1.

FIG. 11B is a perspective view of the support portion of the power toolshown in FIG. 11A.

FIG. 12 is a perspective view of an electrical device, such as a batterycharger, for use with the battery pack shown in FIG. 1.

FIG. 13 is a perspective view of a portion of the battery pack shown inFIG. 1 and illustrating the battery cells and the battery terminalassembly.

FIG. 14 is a top view of the battery cells and the battery terminalassembly shown in FIG. 13.

FIG. 15 is a bottom view of the battery cells and the battery terminalassembly shown in FIG. 13.

FIG. 16 is a front view of the battery cells and the battery terminalassembly shown in FIG. 13.

FIG. 17 is a rear view of the battery cells and the battery terminalassembly shown in FIG. 13.

FIG. 18 is a right side view of the battery cells and the batteryterminal assembly shown in FIG. 13.

FIG. 19 is a left side view of the battery cells and the batteryterminal assembly shown in FIG. 13.

FIG. 20 is a schematic diagram of components of a battery pack, such asthe battery pack shown in FIG. 1.

FIG. 21 is another schematic diagram of components of a battery pack.

FIG. 22 is yet another schematic diagram of components of a batterypack.

FIG. 23 is still another schematic diagram of components of a batterypack.

FIG. 24 is a perspective view of a portion of the battery pack shown inFIG. 1 with portions removed.

FIG. 25 is a perspective view of a portion of the battery pack shown inFIG. 1 with portions removed.

FIG. 26 is a perspective view of a portion of the battery pack shown inFIG. 1 with portions removed.

FIG. 27 is a top view of the portion of the battery pack shown in FIG.26.

FIG. 28 includes views of portions of the battery pack shown in FIG. 26.

FIG. 29 is an exploded perspective view of a portion of the battery packshown in FIG. 1 with portions removed.

FIG. 30 is a rear perspective view of a portion of the battery packshown in FIG. 1 with portions removed.

FIG. 31 is another rear perspective view of the portion of the batterypack shown in FIG. 30.

FIG. 32 is an exploded perspective view of a portion of the battery packshown in FIG. 1 with portions removed.

FIG. 33 is a perspective view of the portion of the battery pack shownin FIG. 32.

FIG. 34 is an enlarged perspective view of a portion of the battery packshown in FIG. 33.

FIG. 35 includes views of portions of the battery pack shown in FIG. 1with portions removed.

FIG. 36 is a rear perspective view of an existing battery pack.

FIG. 37 is a front perspective view of the battery pack shown in FIG.36.

FIG. 38 is a left side view of the battery pack shown in FIG. 36.

FIG. 39 is a perspective view of a portion of the battery pack shown inFIG. 36 and illustrating the battery cells and the battery terminalassembly.

FIG. 40 is a right side view of the battery cells and the batteryterminal assembly shown in FIG. 39.

FIG. 41 is a front perspective view of another battery pack.

FIG. 42 is a right side view of the battery pack shown in FIG. 41.

FIG. 43 is a left side view of the battery pack shown in FIG. 41.

FIG. 44 is a top view of the battery pack shown in FIG. 41.

FIG. 45 is a bottom rear perspective view of the battery pack shown inFIG. 41.

FIG. 46 is a front view of the battery pack shown in FIG. 41.

FIG. 47 is a rear view of the battery pack shown in FIG. 41.

FIG. 48 is a front perspective view of a further battery pack.

FIG. 49 is a right side view of the battery pack shown in FIG. 48.

FIG. 50 is a left side view of the battery pack shown in FIG. 48.

FIG. 51 is a top view of the battery pack shown in FIG. 48.

FIG. 52 is a bottom rear perspective view of the battery pack shown inFIG. 48.

FIG. 53 is a front view of the battery pack shown in FIG. 48.

FIG. 54 is a rear view of the battery pack shown in FIG. 48.

FIG. 55 is a perspective view of a battery pack in use with a firstelectrical device, such as a power tool.

FIG. 56 is a perspective view of a battery pack in use with a secondelectrical device, such as a power tool.

FIG. 57 is a perspective view of a portion of a battery pack andillustrating the battery cells.

FIG. 58 is a perspective view of a portion of a battery pack andillustrating the battery cells, terminals, end caps and circuitry.

FIG. 59 is a rear perspective view of the portion of the battery packshown in FIG. 58.

FIG. 60 is a right side view of the portion of the battery pack shown inFIG. 58.

FIG. 61 is a left side view of the portion of the battery pack shown inFIG. 58.

FIG. 62 is a front view of the portion of the battery pack shown in FIG.58.

FIG. 63 is a rear view of the portion of the battery pack shown in FIG.58.

FIG. 64 is a top view of the portion of the battery pack shown in FIG.58.

FIG. 65 is a perspective view of a portion of a battery pack andillustrating the end caps.

FIG. 66 is a partial side perspective view of a portion of the housingof a battery pack.

FIG. 67 is a partial front perspective view of the portion of thehousing shown in FIG. 66.

FIGS. 68-69 are still further schematic diagrams of components of abattery pack.

FIG. 70 is a partial cross-section of a battery cell.

FIG. 71 is a graph illustrating charging thresholds of Li-based batterypacks.

FIG. 72 is a graph illustrating the performance of two battery packsduring discharge.

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items.

DETAILED DESCRIPTION

A battery pack 30 embodying aspects of the invention is illustrated inFIGS. 1-9. The battery pack 30 can be connectable to electricalequipment, such as, for example, a cordless power tool 34 (shown in FIG.11A) to selectively power the power tool 34. The battery pack 30 can beremovable from the power tool 34 and can be rechargeable by a batterycharger 38 (shown in FIG. 8).

As shown in FIGS. 1-9, the battery pack 30 includes a housing 42 and atleast one rechargeable battery cell 46 (shown in FIGS. 13-19) supportedby the housing 42. In the illustrated construction, the battery pack 30can be a 21V battery pack including five approximately 4.2V batterycells 46 a, 46 b, 46 c, 46 d and 46 e connected in series. In anotherconstruction, the battery pack 30 can be a 28V battery pack includingseven approximately 4.2V battery cells 46. In other constructions, thebattery pack 30 includes at least six battery cells 46 to have a nominalvoltage of approximately 28V. In further constructions (not shown), thebattery pack 30 may have another nominal battery voltage, such as, forexample, 9.6V, 12V, 14.4V, 24V, 28V, 40V, etc., to power the electricalequipment and be charged by the battery charger 38.

In some constructions, the battery pack 30 may include five to tenbattery cells 46. In other constructions, the battery pack 30 mayinclude six to eight battery cells 46. In further constructions, thebattery pack 20 may include two to ten battery cells 46. In someconstructions, the battery cells 46 can have a nominal voltage ofapproximately 3.6V. In other embodiments, the battery cells 46 can havea nominal voltage of approximately 4.2V. In further constructions, thebattery cells 46 can have another nominal voltage, such as, for example,approximately 3.6V, approximately 4.0 V, and approximately 4.3 V. Itshould be understood that, in other constructions (not shown), thebattery cells 46 may have a different nominal cell voltage and/or may beconnected in another configuration, such as, for example, in parallel orin a parallel/series combination.

The battery cells 46 may be any rechargeable battery cell chemistrytype, such as, for example, nickel cadmium (NiCd), nickel-metal hydride(NiMH), Lithium (Li), Lithium-ion (Li-ion), other Lithium-basedchemistry, other rechargeable battery cell chemistry, etc. In theillustrated construction, the battery cells 46 can be lithium-ion(Li-ion) battery cells. For example, the battery cells 46 can have achemistry of Lithium-Cobalt (Li—Co), Lithium-Manganese (Li—Mn) Spinel,Li—Mn Nickel, another lithium metal chemistry, or the like.

In some constructions and in some aspects, such as the constructionshown in FIG. 70, the battery cells 46 can have a Li-ion basedchemistry, such as, for example, Li—Mn Spinel or Li—Co. As shown in FIG.70, the battery cell 46 includes cathode material 705 and anode material710. The cathode material 705 and anode material 710 are separated by aseparator 715. In one construction, the cathode material 705 includesLi—Mn Spinel, the anode material 710 includes lithium graphite and theseparator 715 includes polyethylene polypropylene laminate. In thisconstruction, the Li—Mn Spinel battery cell 46 is typically more stablethan a Li—Co battery cell, which can allow for a larger cell size. Abattery cell 46 of this construction can operate at high currentdischarge rates and does not build large internal resistance duringcycle life.

In some constructions, such as the illustrated construction of FIG. 70,the cell 46 includes one or more safety mechanisms. For example, in oneconstruction, the separator 715 can include a plastic porous separatormaterial that melts between approximately 120° C. and approximately 150°C. When the separator 715 begins to melt, the pores close. This causesthe cell 46 to be disabled.

Also, the cell 46 can include a disconnect device 720, in someconstructions. In these constructions, the disconnect device 720 ispressure activated. If the cell 46 is overcharged, a voltage triggeredpolymer (not shown) within the cell 46 creates gas and pressure. Withenough pressure, the disconnect device 720 will permanently open theelectrical connection to the outside, which disables the cell 46.

Also in some constructions, the cell 46 can include a rupture vent 730.In the instance of large internal pressure buildup (under thermal ormechanical abuse situations), the vent 730 can safely release the gaspressure, such that the cell 46 does not explode.

In one construction of the Li-ion battery cell 46, the workingtemperature range for the Li-ion battery cells 46 is approximately −20°C. to +75° C. As compared to battery cells of other chemistries, theLi-ion battery cells 46 have a wider working temperature range. Forexample, battery cells of NiCd construction do not deliver power as wellas Li-ion cells 46 at higher temperatures, and battery cells of NiMHconstruction do not deliver power as well as Li-ion cells 46 at lowertemperatures. Also, Li-ion battery cells 46 can be lightweight andcompact without sacrificing performance due to high energy densities.Gravimetric energy density for Li-ion battery cells 46 is approximately150 Wh/kg, which can be compared to existing NiCd battery cells. In someconstructions, the gravimetric energy density for Li-ion battery cells46 is approximately 2.5 times as high as existing NiCd battery cells.Volumetric energy density for Li-ion battery cells 46 is approximately390 Wh/L, which can also be compared to existing NiCd battery cells. Insome constructions, the volumetric energy density for Li-ion batterycells 46 is approximately 2.0 times as high as existing NiCd batterycells.

In some constructions and in some aspects, battery cells 46 having aLi-ion based chemistry, such as Li—Co or Li—Mn Spinel, can have higheroperating voltages, lower self discharge and also eliminate the use ofenvironmentally hard elements in the construction of the cells 46, whencompared to existing Nickel based cells, such as NiCd and NiMH. In oneconstruction, a Li-ion battery cell has a nominal voltage ofapproximately 4.0V versus NiCd and NiMH cells having a nominal voltageof approximately 1.2V.

Also, in this construction, a typical self discharge rate for a Li-ionbattery cell 46 is approximately 2% to approximately 5% per month,whereas a typical self discharge rate for a NiCd battery cell isapproximately 15% to approximately 20%.

Further, in this construction, the Li-ion battery cells 46 help toeliminate the environmentally hazardous heavy metals, such as lead,nickel, and cadmium, which are present in other battery types. Theinclusion of these environmentally hazardous metals in other batterytypes requires special recycling methods. In the illustratedconstructions, the Li-ion battery cells 46 do not containenvironmentally hazardous materials. Rather, in these constructions, theLi-ion cells 46 include graphite powder, Li—Mn spinel, carbonatesolvents, lithium hexafluorophosphate, copper, aluminum andpolyvinylidene fluoride (“PVDF”). The cells 46 also include steel,nickel, and an inert material for the can. In these constructions, aLi-ion battery cell 46 does not require any special recycling.

In some constructions, the battery cells 46 can have a chemistry ofLi—Co. In these constructions, the battery cells 46 can be monitored inorder to protect the battery cells 46 from being overdischarged orovercharged, both of which can damage the cells 46 and affect thestability of the cells 46. In some constructions, Li—Co cells 46 mayrequire tighter control during charging and discharging operation thanLi—Mn Spinel cells. For example, in some constructions, the Li—Co cells46 are monitored more closely during charging to prevent overcharging.In these constructions, overcharging of a Li—Co cell 46 may causelithium metal to plate on the anode of the cell 46, may cause lithiummetal to become a powerful reducing agent during plating, may cause thecathode material to become unstable and a strong oxidizing agent, andmay cause excessive heating. Also, lithium metal and the destabilizationof the cathode material can make the Li—Co cell 46 more sensitive tothermal runaway. The graph 800 shown in FIG. 71 helps illustrate thetighter control needed for charging Li—Co cells compared to Li—Mn cells.The curve 805 illustrates the charging threshold for Li—Co cells basedon cell voltage and temperature. The curve 810 illustrates the chargingthreshold for Li—Mn cells based on cell voltage and temperature. Anypoint falling above the respective thresholds 805 and 810 representsthermal runaway. Cells having a Li—Mn based chemistry can have anincreased voltage capacity and still maintain a high temperaturetolerance during charging. Cells having a Li—Co based chemistry shouldbe charged at a lower temperature threshold if the cells increasevoltage capacity.

Similarly, for battery cells 46 having a chemistry of Li—Mn, the cells46 can also be monitored in order to protect the battery cells 46 frombeing overdischarged or overcharged. Examples of systems and methods tomonitor battery cells are described in more detail in U.S. patentapplication Ser. No. 10/720,027, filed Nov. 20, 2003 and Ser. No.11/138,070, filed on May 24, 2005, the contents of all of which arehereby incorporated by reference.

As shown in FIGS. 13-20, in the battery pack 30, each battery cell 46a-46 e can be generally cylindrical and can extend along a cell axis 50a-50 e parallel to the cylindrical outer cell wall. Also, in the batterypack 30, each battery cell 46 can have a cell length 52 which is morethan two times and almost three times the cell diameter 54. In theillustrated construction and in some aspects, each battery cell 46 canhave a diameter of about twenty-six millimeters (26 mm) and a length ofat least about sixty millimeters (60 mm). In some constructions, eachbattery cell 46 can have a length of about sixty-five millimeters (65mm). In some constructions, each battery cell 46 can have a length ofabout seventy millimeters (70 mm). Another example of a battery cell isshown and described in U.S. Pat. No. 6,489,060, the content of which ishereby incorporated by reference.

The battery cells 46 are arranged in a first set 56 of battery cells 46a, 46 b and 46 c and a second set 58 of battery cells 46 d and 46 e. Inthe first set 56, the cell axes 50 a, 50 b and 50 c are parallel to oneanother. In the second set 58, the cell axes 50 d and 50 e are parallelto each other. However, the sets 56 and 58 are arranged so that thebattery cells 46 a, 46 b and 46 c are non-parallel to the battery cells46 d and 46 e. In the illustrated construction, for example, the batterycells 46 a, 46 b and 46 c can be normal to the battery cells 46 d and 46e.

The battery cells 46 are arranged to reduce the heat transfer betweenthe battery cells 46 and to improve the collection and removal of heatfrom the battery cells 46. In this manner, the battery cells 46 may beable to be maintained in an appropriate temperature operating range forlonger durations of use. The battery cells 46 are also arranged toprovide an efficient use of space and to maintain a relatively smallpack size.

As shown in FIGS. 1-4 and 7, the housing 42 can provide a supportportion 60 for supporting the battery pack 30 on an electrical device,such as the power tool 34 or the battery charger 38. In the illustratedconstruction, the support portion 60 provides a C-shaped cross section(see FIG. 7) which is connectable to a complementary T-shaped crosssection support portion on the electrical device.

The battery pack 30 also can include (see FIGS. 1-4, 8-9, 21, 24-25 and30-38) a locking assembly 74 operable to lock the battery pack 30 to anelectrical device, such as, for example, to the power tool 34 and/or toa battery charger. The locking assembly 34 includes locking members 78which are movable between a locked position, in which the lockingmembers 78 engage a corresponding locking member on the electricaldevice to lock the battery pack 30 to the electrical device, and anunlocked position. The locking assembly 74 also includes actuators 82for moving the locking members 78 between the locked position and theunlocked position. The actuators 82 have a large surface for engagementby an operator to provide improved ease of unlocking the lockingassembly 74. Also, the actuators 82 are supported to reduce the grippingforce required to unlock the locking assembly 74.

As shown in FIGS. 30-38, biasing members 83 bias the locking members 78toward the locked position. In the illustrated construction, eachbiasing member 83 is a leaf spring positioned between the actuator 82and the housing 42 to bias the locking member 78 to the locked position.

Each biasing member 83 is fixed between the actuator 82 and the housing42 and operates to retain the actuator 82 (and the locking member 78) ina position and to limit unwanted movement of the actuator 82 (and thelocking member 78) relative to the housing 42. Specifically, the biasingmember 83 limits movement of the actuator 82 (and of the locking member78) in a direction perpendicular to the direction of movement betweenthe locked position and the unlocked position (i.e., upwardly in thecross-sectional views of FIG. 35) to prevent the actuator 82 and/or thelocking member 78 from binding on the housing 42 or from being preventedto move in the desired manner to operate the locking assembly 74.

As shown in FIGS. 32 and 35, the biasing member 83 includes a housingleg 84 engaging the housing 42 to force the biasing member 83 downwardly(in the left cross-sectional view of FIG. 35). The biasing member 83also includes (see the right cross-sectional view of FIG. 35) anactuator leg 85 engaging the actuator 83 to draw and retain the actuator82 (and the locking member 78) in the correct downward position (in thecross-sectional views of FIG. 35) during operation of the lockingassembly 74 and of the battery pack 30.

The battery pack 30 includes (see FIGS. 1-5, 7, 13-14 and 17-20) aterminal assembly 86 operable to electrically connect the battery cells46 to a circuit in the electrical device. The terminal assembly 86includes (see FIGS. 1-3) a positive battery terminal 98, a groundterminal 102, and a sense terminal 106. As schematically illustrated inFIG. 20, the terminals 98 and 102 are connected to the opposite ends ofthe cell or series of cells 46.

The sense terminal 106 can be connected to one or more electricalcomponents, such as an identification component (i.e., a resistor) tocommunicate the identification of a characteristic of the battery pack30, such as, for example, the chemistry of the battery cells 46, thenominal voltage of the battery pack 30, etc., or a temperature-sensingdevice or thermistor to communicate the temperature of the battery pack30 and/or of the battery cell(s) 46. It should be understood that, inother constructions (not shown), the electrical components may be othertypes of electrical components and may communicate other characteristicsor information about the battery pack 30 and/or of the battery cell(s)46. It should also be understood that “communication” and “communicate”,as used with respect to the electrical components, may also encompassthe electrical components having or being in a condition or state whichis sensed by a sensor or device capable of determining the condition orstate of the electrical components.

In some constructions and in some aspects, the sense terminal 106 can beconnected to a circuit 430, as shown in FIGS. 21-23 and 68-69. Thecircuit 430 can be electrically connected to one or more battery cells46, and can be electrically connected to one or more battery terminalsof the terminal block 86. In some constructions, the circuit 430 caninclude components to enhance the performance of the battery pack 30. Insome constructions, the circuit 430 can include components to monitorbattery characteristics, to provide voltage detection, to store batterycharacteristics, to display battery characteristics, to inform a user ofcertain battery characteristics, to suspend current within the battery50, to detect temperature of the battery pack 30, battery cells 46, andthe like, to transfer heat from and/or within the battery 30, and toprovide balancing methods when an imbalance is detected within one ormore battery cells 46. In some constructions and in some aspects, thecircuit 430 includes a voltage detection circuit, a boosting circuit, astate of charge indicator, and the like. In some constructions, thecircuit 430 can be coupled to a print circuit board (PCB) 145. In otherconstructions, the circuit 430 can be coupled to a flexible circuit 445,as discussed below. In some constructions, the flexible circuit 445 canwrap around one or more cells 46 or wrap around the interior of thehousing 42, as discussed below.

In some constructions, the circuit 130 can also include a microprocessor430. The microprocessor 430 can monitor various battery pack parameters(e.g., battery pack present state of charge, battery cell present stateof charge, battery pack temperature, battery cell temperature, and thelike), can store various battery pack parameters and characteristics(including battery pack nominal voltage, chemistry, and the like, inaddition to the parameters), can control various electrical componentswithin the circuit 130, and can conduct communication with otherelectrical devices, such as, for example, a power tool, a batterycharger, and the like. In some constructions, the microprocessor 430 canmonitor each battery cell's present state of charge and can identifywhen an imbalance occurs (e.g., the present state of charge for abattery cell exceeds the average cell state of charge by a certainamount or drops below the average cell state of charge by a certainamount).

In some constructions and in some aspects, the circuit 430 can include avoltage detection circuit 459. In some constructions, the voltagedetection circuit 459 can include a plurality of resistors 460 formingresistor divider networks. As shown in the illustrated construction, theplurality of resistors 460 can include resistors 460 a-d. The pluralityof resistors 460 can be electrically connected to one or more batterycells 46 a-e and to a plurality of transistors 465. In the illustratedconstruction, the plurality of transistors 465 can include transistors465 a-d. In some constructions, the number of resistors included in theplurality of resistors 460 can equal the number of transistors includedin the plurality of transistors 465.

In some constructions, voltage characteristics of the battery pack 30and/or of the battery cells 46 can be read by the microprocessor 440through the plurality of resistors 460 when the microprocessor 440 is inthe active mode. In some constructions, the microprocessor 440 caninitiate a voltage-read event by turning off transistor(s) 470 (i.e.,transistor 470 becomes non-conducting). When the transistor(s) 470 isnon-conducting, the transistors 465 a-d become conducting and voltagemeasurements regarding the battery pack 30 and/or battery cells 46 canbe made by the microprocessor 440. Including the plurality oftransistors 465 in the battery pack 30 can reduce the parasitic currentdraw from the battery pack 30, because the transistors 465 are onlyconducting periodically.

In some constructions, the microprocessor 440 can monitor the voltage ofeach battery cell 46 and balance the cell 46 if an imbalance occurs. Aspreviously discussed, the battery pack 30 can include the plurality ofresistors 460 for providing voltage measurements of the battery cells46. The plurality of resistors 460 are arranged such that themicroprocessor 440 can measure the voltage of each battery cell 46 a-eapproximately at the same time. In some constructions, themicroprocessor 440 detects an imbalance within the battery pack 30 whenone or more cells 46 reach approximately 1 V.

In some constructions and in some aspects, the battery pack 30 mayre-balance the cells 46 when an imbalance has been detected via abalancing circuit 459. In some constructions, the battery pack 30re-balances the battery cells 46 when the battery pack 30 is in adischarging operation or act or when the battery pack 30 is notproviding a discharge current or receiving a charge current. In someconstructions, the balancing circuit 459 can include the plurality ofresistors 460 and the plurality of transistors 465. In someconstructions, the microprocessor 440 disables the battery 30 (e.g.interrupts battery operation, prevents battery operation, etc.) via theswitch 180 when a balanced ratio R between cells 46 is no longerincluded within an acceptable range. After the battery pack 30 isdisabled, the microprocessor 440 determines which cell(s) 46 isimbalanced (the “low voltage cell”).

In some constructions, the microprocessor 440 activates or turns on therespective transistors, such as, for example, transistors 465 a-d, thatare electrically connected to those cells 46 that are not low in presentstate of charge (i.e., cells having a higher present state of chargethan the low voltage cell). The microprocessor 440 begins a controlleddischarge of the high present state of charge cells 46. For example, themicroprocessor will control the small discharge current that will flowfrom the balanced cells 46 through the respective transistors. Themicroprocessor 440 will continue to make voltage measurements of thecells 46 throughout the controlled discharging process. Themicroprocessor 440 will end the controlled discharge process when thepresent state of charge of the higher state of charge cells 46 isreduced to be approximately equal to the previously low voltage cell.

Components of the circuit 430 and of the battery pack 30, such as, forexample, a FET 480, a heat sink 485, a thermistor 450, a fuel gauge 170(including one or more light-emitting diodes 470 a-d), a push-button 460for activating the fuel gauge 470, a microprocessor 440, and the like,are illustrated in more detail in FIGS. 20-29. For some constructionsand for some aspects, these and other additional independent featuresand structure of the battery pack 30 and other operations of the batterypack 30 are described in more detail in U.S. patent application Ser. No.10/720,027, filed Nov. 20, 2003, and Ser. No. 11/138,070, filed May 24,2005.

As shown in FIG. 8, the battery charger 38 is connectable to the batterypack 30 and is operable to charge the battery pack 30. The batterycharger 38 includes a charger housing 122 providing a support portion124, on which the battery pack 30 is supported, and a charging circuit126 (schematically illustrated in FIG. 12) is supported by the housing122 and connectable to a power source (not shown). The charging circuit126 is connectable by a charger terminal assembly 128 to the terminalassembly 86 of the battery pack 30 and is operable to transfer power tothe battery pack 30 to charge the battery cell(s) 46.

In some constructions and in some aspects, the charging circuit 126operates to charge the battery pack 30 in a manner similar to thatdescribed in U.S. Pat. No. 6,456,035, issued Sep. 24, 2002, and U.S.Pat. No. 6,222,343 on Apr. 24, 2001, which are hereby incorporated byreference.

For some constructions and for some aspects, additional independentfeatures, structure and operation of the battery charger 38 aredescribed in more detail in U.S. patent application Ser. No. 10/720,027,filed Nov. 20, 2003, Ser. No. 10/719,680, filed Nov. 20, 2003, Ser. No.11/138,070, filed on May 24, 2005, and Ser. No. 11/139,020, filed on May24, 2005.

The battery pack 30 is connectable to electrical equipment, such as, forexample, the power tool 34 (shown in FIG. 11A), to power the power tool34. The power tool 34 includes a housing 182 supporting an electricmotor 184 (schematically illustrated) which is electrically connected tothe battery pack 30 by (see FIG. 11B) a power tool terminal assembly 186so that the motor 184 is selectively powered by the battery pack 30. Thehousing 182 provides (see FIG. 11B) a support portion 186 on which thebattery pack 30 is supported. The support portion 186 has a generallyT-shaped cross section which is complementary to the C-shaped crosssection of the support portion 60 of the battery pack 30. The supportportion 186 also defines locking recesses 188 (one shown) in which thelocking members 78 are engageable to lock the battery pack 30 to thepower tool 34.

An alternative construction of a battery pack 30A embodying aspects ofthe invention is illustrated in FIG. 10. Common elements are identifiedby the same reference number “A”.

As stated previously, the battery pack 30 can include more or fewerbattery cells 46 than the embodiment shown, and can have a higher orlower nominal voltage than in the constructions shown and described. Forexample, one such construction of a battery pack 30B having a highernominal voltage is shown in FIGS. 41-47. Common elements are identifiedby the same reference number “B”. A further construction of a batterypack 30C is shown in FIGS. 48-54. Common elements are identified by thesame reference number “C”.

Unless specified otherwise, hereinafter, battery pack 30 can refer tothe various constructions of battery pack 30 (e.g., battery pack 30,battery pack 30A, battery pack 30B, and battery pack 30C). Also, unlessspecified otherwise, battery pack 30B can refer to both battery pack 30Band battery pack 30C.

In some constructions, the battery pack 30 can be configured fortransferring power to and receiving power from various electricaldevices, such as, for example, various power tools, battery chargers,and the like. In other constructions, the battery pack 30 can beconfigured for transferring power to various high-power electricaldevices, such as, for example: various power tools and includingelectrically powered tools used in manufacturing and assembly; lawn andgarden equipment and including tools used in agricultural applications;portable lighting, signaling devices, and flashlights; motorizedvehicles including electrically powered scooters, mopeds, motorizedcarts, and the like; vacuum cleaners and other electrically poweredhousehold and commercial applications, tools, and devices; electricallypowered toys; remote-controlled airplanes, automobiles, and othervehicles as well as auxiliary motors and the like. In someconstructions, such as, for example, the constructions illustrated inFIGS. 55 and 56, the battery pack 30 can supply power to various powertools, such as, a driver drill 300, a circular saw 305, band saw (notshown), grinder (not shown) and the like. In some constructions, thebattery pack 30 can power various power tools (including a driver drill300 and a circular saw 305) having high discharge current rates. Forexample, the battery pack 30 can supply an average discharge currentthat is equal to or greater than approximately 20 A, and can have anampere-hour capacity of approximately 3.0 A-h. In other constructions,the battery pack 30 can supply an average discharge current that isequal to or greater than approximately 10 A. In further constructions,the battery pack 30 can supply an average discharge current that isequal to or greater than approximately 15 A. In still furtherconstructions, the battery pack 30 can have a different ampere-hourcapacity, such as, for example, approximately 2.5 A-h or approximately2.8 A-h for battery cells 46 arranged in series or approximately 1.3 A-hor approximately 1.5 A-h for battery cells 46 arranged in parallel.

In some constructions, the battery pack 30, such as battery pack 30B,can include seven battery cells 346 a-g (shown in FIG. 57). In someconstructions, the battery cells 346 a-g can be similar to battery cells46 a-e included in the battery pack 30. In some constructions, thebattery cells 346 a-g can differ from battery cells 46 a-e in weight,size, nominal voltage, chemistry, and the like. For example, in oneconstruction, the battery cells 346 a-g can have a cell chemistry ofLi-ion, such as, for example, Li—Mn spinel, Li—Mn nickel, or Li—Co. Insome constructions, each cell 346 a-g can have a nominal voltage ofapproximately 3.6 V. In other constructions, each cell 346 a-g can havea nominal voltage of approximately 4 V, and in further constructions,each cell 346 a-g can have a nominal voltage of approximately 4.2 V. Insome constructions, the battery pack 30B can include seven battery cells346 a-g, and can have a nominal voltage of approximately 28 V. In otherconstructions, the battery pack 30B can include seven battery cells 346a-g, and can have a nominal voltage of approximately 25 V.

The battery cells 346 a-g can also be electrically connected in anysuitable manner, such as, for example, in a serial arrangement, aparallel arrangement, a partial serial arrangement (e.g., some of thebattery cells 346 a-g are connected in a serial arrangement), a partialparallel arrangement (e.g., some of the battery cells 346 a-g areconnected in a parallel arrangement), a combination of a serial,parallel, partial serial or partial parallel arrangement. In oneconstruction, the battery cells 346 a-g are electrically connected in aserial arrangement. The battery cells 346 a-g can be electricallyconnected via conductive straps 450. For example, a conductive strap 450can connect the negative end of the first battery cell 346 a to thepositive end of the second battery cell 346 b. Also, another conductivestrap 450 can connected the negative end of the second battery cell 346b to the positive end of the third battery cell 346 c.

As shown in FIGS. 58-65, the battery pack 30, such as battery pack 30B,can also include an end cap arrangement 505. In some constructions, theend cap arrangement can be used for spacing the battery cells 346. Theend cap arrangement 505 includes a first end cap 510 and a second endcap 515. The first and second end caps 510 and 515 can be connected by aconnecting portion 520. In some constructions, the connecting portion520 can be a hinge. In some constructions, the end cap arrangement 505does not include the connecting portion 520. Each end cap 510 and 515can partially define one or more cavities 530 (shown in FIG. 65). Theend of a battery cell 346 can be positioned within a cavity 530. In theillustrated construction, the first end cap 510 and the second end cap520 each include seven cavities 530 a-g for positioning seven batterycells 346 a-g, respectively.

In the illustrated construction, the first end cap 510 is positioned ata first end 490 (shown in FIG. 57) of the arrangement of battery cells346, and the second end cap 515 is positioned at the second end 495 ofthe arrangement of battery cells 346. As mentioned previously, each endof each battery cell 346 a-g can be positioned within the respectivecavities 530 a-g of the first and second end cap 510 and 515. Each endcap 510 and 515 can define the cavities 530 a-g in order to create gapsor spaces between the battery cells 346 when the battery cells 346 arepositioned within the cavities 530. This can allow for greater heatdissipation within the battery pack 30B by allowing air to circulatethrough the gaps and spaces between the cells 346.

In some constructions, the first end cap 510 and the second end cap 515can further define apertures 450. The apertures 450 can receive theconductive straps 450 for electrically connecting one battery cell 346to another battery cell 346.

In some constructions and in some aspects, the end cap arrangement 505can also include a flexible circuit 445. In some constructions, theflexible circuit 445 can be integral with either the first end cap 510,the second end cap 515, the connecting portion 520, or a combination. Inother constructions, the end cap arrangement 505 can define one or moreareas for supporting the flexible circuit. In further constructions, theflexible circuit 445 can be secured to the end cap arrangement 505. Asshown in the illustrated construction, the flexible circuit 445 canpartially wrap around the battery cells 346.

In the construction shown, the end cap arrangement 505 can include aconnector 560 for electrically connecting the flexible circuit 445 tothe PCB 145B. In this construction, the PCB 145B and the flexiblecircuit 445 each can include a portion of the circuit 430 included inthe battery pack 30B.

In some constructions and in some aspects, the battery pack 30 caninclude cushion members or “bumpers” 640. As shown in FIGS. 66 and 67,the interior face 645 of the battery housing 42B can include one or morecushion members 640. In some constructions, the cushion members 640 canbe integral with the housing 42B. In other constructions, the cushionmembers 640 can be attached or secured to the interior face 645 of thehousing 42B. In further constructions, the cushion members 640 can beconnected to one or more battery cells 346 or to the end cap arrangement505 partially surrounding the battery cells 346. In some constructions,the cushion members 645 can absorb energy during impact and protect thebattery cells 346 during impact by limiting the amount of energytransferred to the cells 346. The cushion members 645 can include anythermoplastic rubber such as, for example, polypropylene RPT 100 FRHI(e.g., flame retardant-high impact).

FIG. 72 depicts a graph 900 illustrating the performance of two batterypacks during a cutting test. The test includes continuous cross cuttingof 2″×21″ pine with a cordless circular saw. In this construction, afirst battery pack (not shown) includes five (5) Li—Mn Spinel batterycells, each having a 4.0 V nominal voltage and a 3 Ah capacity. Also inthis construction, a second battery pack (not shown) includes fifteen(15) NiCd battery cells, each having a 1.2 V nominal voltage and a 2.4Ah capacity. In this construction, the first battery pack is capable ofapproximately 40% more work and weighs approximately 35% less than thesecond battery pack. The graph 900 illustrates the pack voltage for eachbattery pack. The first curve 905 illustrates the voltage for the firstbattery pack. The second curve 910 illustrates the voltage for thesecond battery pack. As shown from the graph 900, the voltage 905 of thefirst battery pack remains steady and flat longer than the voltage 910of the second battery pack. Thus, the first battery pack, which includesLi-ion cells, is able to maintain its voltage during discharge, asopposed to the second battery pack, which includes NiCd cells.

One or more independent features or independent advantages of theinvention will be set forth in the claims.

1. A battery pack configured to be interfaced with an electrical device,comprising: a battery pack housing; at least three terminals including apositive terminal, a negative terminal, and a sense terminal, the atleast three terminals configured to be interfaced with correspondingterminals of the electrical device; a plurality of rechargeable batterycells arranged within and supported by the housing, each of the batterycells having a respective state of charge, power being transferablebetween the battery cells and the electrical device; a balancing circuitconfigured to detect a charge imbalance among the battery cells based ona plurality of voltages of the plurality of rechargeable battery cellsdetected at substantially the same time; and a control circuitconfigured to balance the battery cells upon detection of the chargeimbalance by the balancing circuit, the control circuit including amicroprocessor.
 2. The battery pack of claim 1, wherein the balancingcircuit includes a plurality of resistors.
 3. The battery pack of claim1, wherein the balancing circuit includes a plurality of transistors. 4.The battery pack of claim 1, wherein the control circuit is furtherconfigured to identify a battery cell among the plurality of batterycells that is imbalanced.
 5. The battery pack of claim 4, wherein theidentified battery cell is a low voltage cell.
 6. The battery pack ofclaim 1, wherein the battery pack is configured to balance the batterycells when the battery pack is in a discharging operation.
 7. Thebattery pack of claim 1, wherein the microprocessor is configured todisable the battery pack via a switch when a balanced ratio between thebattery cells falls outside a predetermined range.
 8. The battery packof claim 7, wherein the microprocessor is configured to interruptoperation of the battery pack when the balanced ratio between thebattery cells falls outside the predetermined range.